1. Field of the Invention
The present invention relates to a structure of a turbine wheel used for a turbine of a gas turbine or an exhaust gas turbocharger; the invention particularly relates to a turbine wheel having what they call a scallop part, namely, a wavy-edge part on the back side of the turbine wheel, the scallop part being formed by cutting out of the back side of a radial turbine, so as to leave the blade parts.
2. Background of the Invention
As a rule, there are two types of turbine wheel in the radial turbine: a turbine wheel (shown in FIG. 8(a)) provided with what they call a scallop part 01, namely, a wavy-edge part on the back side (the rear side) of the turbine wheel, the scallop part being formed by cutting out of the back side of a radial turbine; and, a turbine wheel (shown in FIG. 8 (b)) provided with a back side wall (part) 02 of a circular disk shape, the scallop part being not formed on the back side of the turbine wheel.
The turbine wheel with the scallop part is advantageous in rotational inertia reduction, material cost reduction and thermal stress reduction; however, the efficiency of the turbocharger in which the turbine wheel with the scallop part is installed is inclined to be inferior to the efficiency of the turbocharger that is provided with the turbine wheel with not the scallop part but the back side wall 02.
Nevertheless, in response to the ever stricter regulations regarding exhaust gas emission and energy conservation in recent years, the quicker response performance regarding the turbocharger is being required and a reappraisal of the turbine wheel with the scallop part is being performed.
Patent Reference 1 (JP2000-170541) proposes the conventional technology regarding the turbine wheel with not the scallop part but the back side wall, whereas Patent Reference 2 (JP1998-131704) and Patent Reference (JP2003-201802) propose the conventional technology regarding the turbine wheel with the scallop part.
According to the technology disclosed in Patent Reference 1, the outer periphery diameter of the back side wall of the circular disk shape approximately agrees with the diameter of the blade parts so that the strength or rigidity of the turbine rotor is enhanced. Further, since the back side wall part blocks the gap between the turbine rotor and the wall part on the casing side (on the stator side), the leakage of the working fluid toward the rear side of the turbine rotor (i.e. hereby the turbine wheel) is prevented; thus, the leakage loss is reduced.
Further, according to Patent Reference 2, as depicted in FIG. 9, a turbine wheel 03 is provided with a plurality of blade parts 04; a scallop part 06 is formed in a main wall 05 (that is almost the same as the back side wall in Patent Reference 2) inside the back side of the blade parts 04 of the turbine wheel 03. Each blade part (in a cross-section profile regarding the blade part) includes a suction surface 012, a minimum radius part 08 and a pressure surface (a pressure surface) 013; the minimum radius part 08 between a blade part 04 and the adjacent blade part 04 is biased toward the suction surface side 012 so that the suction surface 012 of a blade part and the pressure surface 013 of the blade part are unsymmetrically arranged with regard to the blade part (namely, unsymmetrical with regard to the left side and the right side of the blade camber line). Thus, the angle which the pressure surface 013 and the front side vertical plane of the main wall 05 form becomes acuter, the front side vertical plane of the main wall 05 being vertical to the rotation axis. Thus, in this area of the acuter corner, the factor regarding the energy dissipation loss is increased so that the fluid flow streaming from the pressure surface side 13 to the suction surface side 12 through the rear side of the blade part is constrained; and, the efficiency deterioration due to the leakage is constrained.
Further, according to Patent Reference 3, as depicted in FIG. 10, a turbine wheel 020 is provided with a plurality of blade parts 24; a scallop part 021 is formed in a circular main wall 022 between the back side of the blade part 024 and the adjacent the blade part 024; a minimum radius part is formed on the scallop hem part (the scallop profile) so that the distance from the center of the circular main wall 022 to the minimum radius part is minimum. Further, the scallop part includes a suction surface side surface 026 of a blade part 024 and a (positive) pressure side surface 028 of the adjacent blade part 024; and, the minimum radius part is located on the (positive) pressure surface side with regard to a middle location in a hoop direction between the (positive) pressure side surface and the suction surface side surface. Thus, the scallop part 021 is formed unsymmetrically between the (positive) pressure side surface 028 and the suction surface side surface 026. And, in this way, the corner vortex flow is constrained in the neighborhood along the suction surface side surface 026 of the scallop part 021, so that turbine efficiency is enhanced.